# Tx\_ofdm **URL:** \ **Category:** GNU Radio\ **Created:** [May 22, 2014, 7:33pm UTC](https://www.ruby-forum.com/t/tx-ofdm/241305 "2014-05-22T19:33:18Z")\ **Posts on this page:** 7\ **Page:** 1
**Author:** ![Detlef\_R](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/d/58f4c7/32.png) [@Detlef\_R](https://www.ruby-forum.com/u/Detlef_R)\ **Post date:** [May 22, 2014, 7:33pm UTC](https://www.ruby-forum.com/t/tx-ofdm/241305/1 "2014-05-22T19:33:18Z")
Hi everyone: I have already understand most part of the tx\_ofdm.grc.But the some digital i can’t calculate accurately. 1.stream\_to\_tagged\_stream: Packet length=96 bytes.? 2.And ofdm\_carrier\_allocator:FFT\_length=64(but 48 occupied carrier+4 pilot carriers!=64)? Thank you in advance.Thank you. Best regards ---
**Author:** ![xianda](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/x/b38774/32.png) [@xianda](https://www.ruby-forum.com/u/xianda)\ **Post date:** [May 22, 2014, 10:14pm UTC](https://www.ruby-forum.com/t/tx-ofdm/241305/2 "2014-05-22T22:14:22Z")
IIRC, the tx and rx files are just a very general implementation of the OFDM model. Here’s my understanding: - Packet length refers to the data necessary to produce n symbols (48 data carriers, 2 symbols = 96). - The number of total carriers in the systems equals the FFT length. In this case, 48 data carriers and 4 pilots, 52 total and the rest are set to 0. I hope im sure about this, if anyone is able to confirm this just to be precise. [http://gnuradio.org/doc/doxygen/page\_ofdm.html](http://gnuradio.org/doc/doxygen/page_ofdm.html) [http://gnuradio.org/doc/doxygen/page\_packet\_data.html](http://gnuradio.org/doc/doxygen/page_packet_data.html) – View this message in context: [http://gnuradio.4.n7.nabble.com/tx-ofdm-tp48430p48432.html](http://gnuradio.4.n7.nabble.com/tx-ofdm-tp48430p48432.html) Sent from the GnuRadio mailing list archive at [Nabble.com](http://Nabble.com). ---
**Author:** ![xianda](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/x/b38774/32.png) [@xianda](https://www.ruby-forum.com/u/xianda)\ **Post date:** [May 22, 2014, 10:35pm UTC](https://www.ruby-forum.com/t/tx-ofdm/241305/3 "2014-05-22T22:35:19Z")
On 05/22/2014 10:12 PM, eontool wrote: > IIRC, the tx and rx files are just a very general implementation of the OFDM > model. > > Here’s my understanding: > > - Packet length refers to the data necessary to produce n symbols (48 data > carriers, 2 symbols = 96). Er, no. This is the number of bytes per packet. It’s 96 because we add 4 bytes for CRC, and then the packet number is a round value. At 100 bytes per packet, the payload is 800 bits. With BPSK, that would be 17 OFDM symbols (plus 1 OFDM symbol for header, and 2 for preamble). You can set whatever here, but remember that the stock equalizers don’t do a good job on long packets. Also, the length can vary per packet. > - The number of total carriers in the systems equals the FFT length. > In this case, 48 data carriers and 4 pilots, 52 total and the rest are set > to 0. This is correct (compare 802.11a standard). M ---
**Author:** ![xianda](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/x/b38774/32.png) [@xianda](https://www.ruby-forum.com/u/xianda)\ **Post date:** [May 23, 2014, 2:47am UTC](https://www.ruby-forum.com/t/tx-ofdm/241305/4 "2014-05-23T02:47:23Z")
Hi: Thank you so much. 1.But from “chunks to symbols” I see you use the bpsk and qpsk both.Right?What is the relationship between 800 bits and 17 symbols?Namely,how to calculate the 17 symbols? 17\*64=1088.Can you explain? 2.I have looked at your ppt about the ofdm.You say now have two version ofdm.Can you give me some links to them?Or some advices?Thank you. Best regards ---
**Author:** ![xianda](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/x/b38774/32.png) [@xianda](https://www.ruby-forum.com/u/xianda)\ **Post date:** [May 23, 2014, 10:42am UTC](https://www.ruby-forum.com/t/tx-ofdm/241305/5 "2014-05-23T10:42:32Z")
On 05/23/2014 02:29 AM, xianda wrote: > Hi: > Thank you so much. > 1.But from “chunks to symbols” I see you use the bpsk and qpsk > both.Right?What is the relationship between 800 bits and 17 symbols?Namely,how to > calculate the 17 symbols? > 17\*64=1088.Can you explain? 1 OFDM symbol @ BPSK == 48 bits in this configuration. ceil(800/48) == 17. > ``` > 2.I have looked at your ppt about the ofdm.You say now have two version > > ``` ofdm.Can you give me some links to them?Or some advices?Thank you. They’re both in the tree. If you start working off of rx\_ofdm an tx\_ofdm you’re already using the new stuff; no need to worry about this. M ---
**Author:** ![xianda](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/x/b38774/32.png) [@xianda](https://www.ruby-forum.com/u/xianda)\ **Post date:** [May 23, 2014, 11:35am UTC](https://www.ruby-forum.com/t/tx-ofdm/241305/6 "2014-05-23T11:35:42Z")
Hi Martin: Thank you for your reply. But, 1.I see the virual Source(Stream ID:Payload Bits)-\>Chunks to Symbols(Symbol Table(-0.707…))(modulate:qpsk)?When I input 800 bits to the Packet Header Generator,then chunks to symbols.How many symbols generate in this way?When I input 800 bits to the Repack Bits,then chunks to symbols.How many symbols generate in this way?Thank you very much. 2.I also see another ofdm in the old version of gnuradio.And the module structure is attached.And the connect code in python: self.connect((self,0), self.chan\_filt) # filter the input channel self.connect((self,0), self.ofdm\_sync) # into the synchronization alg. self.connect((self.ofdm\_sync,0), self.nco, (self.sigmix,1)) # use sync freq. offset output to derotate input signal self.connect((self,0), (self.sigmix,0)) # signal to be derotated self.connect(self.sigmix, (self.sampler,0)) # sample off timing signal detected in sync alg self.connect((self.ofdm\_sync,1), (self.sampler,1)) # timing signal to sample at ``` self.connect((self.sampler,0), self.fft_demod) # send ``` derotated sampled signal to FFT self.connect((self.sampler,1), (self.ofdm\_null,0)) # send timing signal to decoder self.connect(self.fft\_demod, (self.ofdm\_null,1)) # send freq-domain samples to decoder ``` self.connect(self.fft_demod, (self.ofdm_frame_acq,0)) # find ``` frame start and equalize signal self.connect((self.sampler,1), (self.ofdm\_frame\_acq,1)) # send timing signal to signal frame start self.connect((self.ofdm\_frame\_acq,0), (self,0)) # finished with fine/coarse freq correction, self.connect((self.ofdm\_frame\_acq,1), (self,1)) # frame and symbol timing, and equalization And someone tell me the new version of gnuradio is just expand to the old version.Can you tell me how much parts adjust?And the main code of gnuradio is in gnuradio/gr-digital/examples/ofdm?And whether other code needed?Where are they? 3.And i use two usrps to run the tx\_ofdm.grc and rx\_ofdm.grc.The adjust code I also attached. But in the transmit,so many U U U display.Why?And in the receive part,I also encouter problem.Can you help me explain it? Thank you very much.I see the ofdm part many days.And the 3 questions I encounter.Thank you.Sorry for my poor English if you can’t understand.Thank you in advance.Thank you for your patience. Best regards ---
**Author:** ![xianda](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/x/b38774/32.png) [@xianda](https://www.ruby-forum.com/u/xianda)\ **Post date:** [May 23, 2014, 12:09pm UTC](https://www.ruby-forum.com/t/tx-ofdm/241305/7 "2014-05-23T12:09:02Z")
On 05/23/2014 11:33 AM, xianda wrote: > Hi Martin: > Thank you for your reply. > But, > 1.I see the virual Source(Stream ID:Payload Bits)-\>Chunks > to Symbols(Symbol Table(-0.707…))(modulate:qpsk)?When I input 800 bits > to the Packet Header Generator,then chunks to symbols.How many symbols > generate in this way?When I input 800 bits to the Repack Bits,then > chunks to symbols.How many symbols generate in this way?Thank you very much. I’m pretty sure I’ve answered all of these questions already. > ``` > 2.I also see another ofdm in the old version of > > ``` > > gnuradio.And the module structure is attached.And the connect code in > python: Just ignore that. The new stuff is a complete rewrite. > ``` > 3.And i use two usrps to run the tx_ofdm.grc and > > ``` > > rx\_ofdm.grc.The adjust code I also attached. But in the transmit,so many > U U U display.Why?And in the receive part,I also encouter problem.Can > you help me explain it? Your computer is too slow. M